Black Hole's Mystery 'Wave' Surprises Scientists

Below:

Next story in Space

Astronomers studying an unusual black hole system have spotted a
never-before-seen structure in the disk of matter encircling the
system.

Swift J1357.2, an X-ray binary system that regularly emits
outbursts of high energy, consists of a
black hole slowly consuming its companion star. Matter from
the doomed star falls into the accretion disk, which surrounds
the black hole, feeding it dust and gas.

While observing the system, a team of scientists noticed an
unusual vertical feature traveling through the material.

"It's the first time we can resolve such [a] structure in an
accretion disk, and it might be ubiquitous in X-ray binaries
during the outburst state," Jesus Corral-Santana, of the
Astrophysical Institute of the Canary Islands in Spain, told
SPACE.com by email. [ The
Strangest Black Holes in the Universe ]

A hidden structure

The black hole contained in Swift J1357.2 is one of the millions
of stellar black holes that dot the Milky
Way galaxy.

About three times as massive as
the sun, the behemoth likely formed when a single star
collapsed inward on itself. The resulting, city-sized body packed
a great deal of mass into a tiny package, creating a strong
gravitational pull on nearby dust and gas.

Located in the Virgo
constellation, approximately 4,900 light-years from Earth,
Swift J1357.2 also contains a small companion star, which has
only a quarter the mass of the sun. This companion star orbits
the pair's center of mass every 2.8 hours, one of the shortest
known orbital periods for such systems.

The black hole
pulls material from the companion star into its accretion
disk, occasionally emitting the X-ray bursts that enabled
scientists to find this otherwise hard-to-spot system,
researchers said.

Corral-Santana and his team took hundreds of optical images of
the system using the Isaac Newton and the William Herschel
Telescopes, both of which are in the Canary Islands. Studying the
light produced by the accretion disk, the researchers noticed a
periodic dimming in the system, sometimes occurring over the
course of only a few seconds.

"Since the orbital period of the system is 2.8 hours, those dips
cannot be produced by eclipses of the companion star. They are
much faster," Corral-Santana said. "Therefore, they must be
produced by a hidden structure placed very close to the black
hole, in the inner accretion disk."

The new find can only been seen in the outer, optical portion of
the accretion disk, not on the inside, where X-ray bursts
originate. The X-ray emission, which shows no periodic variation,
unlike its optical counterpart, indicated a vertical structure
was hiding the black hole, Corral-Santana said.

Rather than appearing at a set, predictable time, the structure
shows up over a steadily increasing period, indicating a
wave-like movement through the accretion disk.

"It is a wave produced in the accretion disk, moving outward,"
Corral-Santana said, "like the wave produced when a stone is
dropped in calm water."

The missing population

The wave-like feature also provides information about the
orientation of the black hole.

Objects in space face Earth at a variety of angles, or
inclinations. They can be seen edge-on, face-on or somewhere in
between. Swift J1357.2 is the only one of 50 suspected similar
black-hole systems found with an edge-on accretion disk — what
scientists call a high inclination. However, astronomers think
approximately 20 percent of these systems should provide such a
perspective.

In order to see the wave-like structure in the accretion disk,
scientists must have such an edge-on view of the disk, or one
close to it. A view from a lower inclination, closer to face-on,
would not reveal the sudden rises and falls in the total light
coming from the system.

"Swift J1357.2 is the prototype of the hitherto missing
population of high-inclination black holes in transient X-ray
binaries," Corral-Santana said.

Because Swift J1357.2 is the first such system to allow such an
edge-on view, the presence of the vertical structure takes on an
added significance. No signs of such structures appear in other
similar systems, but that could result simply from their
unfortunate angles. Such structures could in fact exist in other,
previously discovered transient X-ray binary systems, hidden only
by their observational angles.

The findings were published online today (Feb 28) in the journal
Science.